Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. MRAM is non-volatile and so can maintain memory content when the memory device is not powered. MRAM data is stored by magnetoresistive elements. Generally, the magnetoresistive elements in an MRAM cell are made from two magnetic regions, each of which accepts and sustains magnetization. The magnetization of one region (the “pinned region”) is fixed in its magnetic orientation, and the magnetization orientation of the other region (the “free region”) can be changed. Thus, a programming current can cause the magnetic orientations of the two magnetic regions to be either parallel, giving a lower electrical resistance across the magnetoresistive elements (which may be defined as a “0” state), or antiparallel, giving a higher electrical resistance across the magnetoresistive elements (which may be defined as a “1” state) of the MRAM cell. The switching of the magnetic orientation of the free region and the resulting high or low resistance states across the magnetoresistive elements provide for the write and read operations of the typical MRAM cell.
One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell. A conventional STT-MRAM cell may include a magnetic cell core, which may include a magnetic tunnel junction (MTJ) or a spin valve structure. An MTJ is a magnetoresistive data storing element including two magnetic regions (one pinned and one free) and a non-magnetic, electrically insulating region in between, which may be accessed through data lines (e.g., bit lines), access lines (e.g., word lines), and an access transistor. A spin valve has a structure similar to the MTJ, except a spin valve has a conductive region in between the two magnetic regions.
In operation, a programming current may flow through the access transistor and the magnetic cell core. The pinned region within the cell core polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current interacts with the free region by exerting a torque on the free region. When the torque of the spin-polarized electron current passing through the core is greater than a critical switching current density (Jc) of the free region, the torque exerted by the spin-polarized electron current is sufficient to switch the direction of the magnetization of the free region. Thus, the programming current can be used to cause the magnetization of the free region to be aligned either parallel to or antiparallel to the magnetization of the pinned region, and, when the magnetization of the free region is switched between parallel and antiparallel, the resistance state across the core is changed.
The free regions and pinned regions of conventional STT-MRAM cells exhibit magnetization orientations that are horizontal, also known as “in-plane,” with the width of the regions. Accordingly, the magnetization orientations are parallel (or antiparallel) to a plane defined by a primary surface of a substrate supporting the STT-MRAM cell. These wide, in-plane STT-MRAM cells have large footprints, making scaling of the cells below twenty-five nanometers a challenge.
Perpendicularly oriented STT-MRAM cells may require smaller cell widths than in-plane STT-MRAM cells, accommodating greater cell packing. Also, the associated perpendicular magnetizations (also known in the art as perpendicular magnetic anisotropy (“PMA”)) of perpendicularly oriented STT-MRAM cells may have greatly reduced required switching voltage compared to an in-plane STT-MRAM cell. Therefore, efforts have been made to faun perpendicularly oriented (“out-of-plane”) STT-MRAM cells in which the pinned regions and the free regions exhibit vertical magnetization orientations. However, finding and implementing suitable materials and designs for the cell core to achieve the vertical magnetization orientations has been a challenge.